1. Field of the Invention
The present invention concerns a one-piece leak-proof battery. A one-piece storage battery has a single casing which is prismatic in shape comprising chambers separated by walls in which the elements of the storage battery are placed. Such batteries generally have capacities which are in the range 10 Ah to 250 Ah.
2. Description of the Prior Art
"Leak-proof" type valve storage batteries do not allow gas exchange with the surroundings under normal operation. Until now, the majority of leak-proof type batteries have principally been intended for incorporation into portable devices. They are thus small and of limited capacity (.ltoreq.10 Ah). In order to protect the user against any over-pressure, they are preferably cylindrical in shape and the casing is normally metal. Such a casing is expensive and heavy, limiting the energy per unit mass of the battery.
Recently, the demand for batteries with a greater capacity and a higher energy per unit weight has grown. Batteries with the best characteristics are prismatic in shape and have a polymeric material casing. A "one-piece" battery is of great benefit from this viewpoint. Sealing is then a problem, however, in particular when there is a moderate internal over-pressure. A principal condition for sealing a battery is the intrinsic impermeability to liquids and gases of the material constituting the casing.
Loss of part of the electrolyte or of water or of the organic solvent forming the electrolyte base causes an inevitable reduction in battery performance. As an example, if the material used to form the casing is not impermeable to both oxygen and hydrogen, this results in a loss of water, and thus a reduction in battery life, but if the material is only permeable to one of these gases, the operating profile of the battery becomes unbalanced.
The first batteries of the above type had a polypropylene (PP) casing and were used in railroad applications in particular. Their gas permeability has been found to be unsatisfactory.
A battery casing intended particularly for aviation applications has been produced from polyamide 11 (PA11) with chemical formula (--NH--(CH.sub.2).sub.10 --CO--).sub.n obtained by polycondensation of amino-undecanoic acid. However, while its impermeability to gas is better than that of PP, PA absorbs liquids, in particular water and glycol. Further, that material is expensive and raises a resource problem because of its plant origin (castor bean).
When the battery is provided with a cooling system circulating a heat transfer liquid along the walls of the battery, absorption of moisture by the material of the box can cause a loss of electrical insulation between the electrochemical assembly and the cooling liquid. If the electrical insulation reaches a low value, the battery user may be endangered.
The aim of the present invention is to propose a leak-proof one-piece battery with improved liquid and gas impermeability. More particularly, the invention proposes a one-piece battery with a gas permeability of less than 10% of the gas permeability of PP.